1. Field of the Invention
The present invention relates to a seat track mechanism for a vehicle such as an automobile. The seat track mechanism supports a vehicle seat and allows the vehicle seat to be moved forward and rearward for adjustment.
2. Description of the Related Art
A conventional seat track mechanism in which spherical rotators (balls) are installed between a pair of lower rails (lower tracks) and a pair of upper rails (upper tracks) to smooth the sliding movement of the upper rails relative to the lower rails is known in the art. This type of seat track mechanism is disclosed in Japanese unexamined patent publication No. 2005-47347.
FIG. 16 shows a cross sectional view of a conventional seat track mechanism by way of example, taken along a vertical plane orthogonal to the sliding direction of an upper rail (upper track). A lower rail 40 that is fixed to a vehicle floor (floor surface) is in the shape of a hollow box, the top of which is open. The lower rail 40 includes a bottom portion 40a, a pair of (left/right) side wall portions 40b, a pair of inward-extending flanges 40c and a pair of downward-extending flanges 40d, wherein the pair of side wall portions 40b extend upwardly from the lateral edges of the bottom portion 40a, the pair of inward-extending flanges 40c extend inwardly from the top edges of the side wall portions 40b, and the pair of downward-extending flanges 40d extend downwardly from the inner edges of the pair of inward-extending flanges 40c. The upper rail 41 includes a top portion 41a, a pair of (left/right) vertical wall portions 41b, a pair of (left/right) first inclined portions 41c and a pair of (left/right) second inclined portions 41d, wherein the pair of vertical wall portions 41b extend downwardly from the lateral edges of the top portion 41a, the pair of first inclined portions 41c extend obliquely toward the adjacent side wall portions 40b from the bottom edges of the pair of vertical wall portions 41b, the pair of second inclined portions 41d extend obliquely upwards from the outer edges of the pair of first inclined portions 41c. Each of the pair of first inclined portions 41c is inclined upwardly in a direction from the boundary between the first inclined portion 41c and the associated vertical wall portion 41b (i.e., from the widthwise center of the upper rail 41) toward the adjacent side wall portion 40b. Each of the pair of second inclined portions 41d is inclined upwardly in a direction away from the boundary between the second inclined portion 41d and the associated first inclined portion 41c toward the widthwise center of the upper rail 41. Two pairs (left/right pairs) of balls 42 and 43 are installed between the lower rail 40 and the upper rail 41, respectively. Each ball 42 is inscribed in the boundary between the bottom portion 40a and the adjacent side wall portion 40b of the lower rail 40 and also in contact with the adjacent first inclined portion 41c of the upper rail 41. Each ball 43 is inscribed in the boundary between the adjacent side wall portion 40b and the adjacent inward-extending flanges 40c of the lower rail 40 and also in contact with the adjacent second inclined portion 41d of the upper rail 41. Accordingly, in the seat track mechanism shown in FIG. 16, the four balls 42 and 43 are installed to be inscribed in four corners of the inner surface of the box-shaped lower rail 40 to be pressed against these four corners so that the upper rail 41 is indirectly supported by the lower rail 40 via the four balls 42 and 43 at the four corners. If a moving force in the forward/rearward direction is given to the upper rail 41, the upper rail 41 moves in the forward/rearward direction relative to the lower rail 40 while rolling (rotating) the four balls 42 and 43.
In this type of seat track mechanism using balls in the manner as shown in FIG. 16, there is a problem with the working accuracies of the lower rail 40 and the upper rail 41 being apt to exert an influence on the positional accuracy and the sliding performance of the upper rail 41. Specifically, the heightwise position of the upper rail 41 with respect to the lower rail 40 is determined by the engagements of the left/right first inclined portions 41c with the two balls 42; however, if the points of contact of the pair of first inclined portions 41c relative to the balls 42 in the rail widthwise direction (left/right direction with respect to FIG. 16) vary, the heightwise position of the upper rail 41 varies because the left/right first inclined portions 41c are inclined to both the horizontal and vertical directions. In addition, since a force urging each ball 42 to be held at a balancing position relative to the associated first inclined portion 41c is produced, the upper rail 41 easily tilts when the bilaterally symmetrical shapes of the left/right first inclined portions 41c relative to the rail widthwise direction are not properly maintained or when a slanting (rotational) force along the first inclined portions 41c acts on the upper rail 41. Additionally, the downward load acting on the upper rail 41 is received by the pair of first inclined portions 41c and the balls 42; however, a component force urging the pair of vertical wall portions 41b to approach each other along the inclinations of the left/right first inclined portions 41c is produced if a downward load is imposed on the upper rail 41 from the top portion 41a when, e.g., an occupant sits in the seat on the upper rail 41. If the upper rail 41 is deformed by this component force, there is a possibility that the sliding load becomes too light or play occurs between the upper rail 41 and the lower rail 40 and between the upper rail 41 and the balls 42 and 43.
Additionally, not only a downward load but also an upward load acts on the seat track mechanism via a seatbelt anchor and the like. In the seat track mechanism shown in FIG. 16, a load in a direction to pull up the upper rail 41 is received by the balls 43 via the left/right second inclined portions 41d. Since the second inclined portions 41d are inclined to both the horizontal and vertical directions, a component force in a horizontal direction is produced due to the inclination of the left/right second inclined portions 41d if a load in the upward direction with respect to FIG. 16 acts on the upper rail 41. This component force acts on the upper rail 41 to urge the pair of vertical wall portions 41b to approach each other and also acts on the lower rail 40 to urge the pair of side wall portions 40b to move away from each other. If the positions of the upper rail 41 and the lower rail 40 change according to this component force, the force to hold the balls 42 and 43 becomes weak, so that the upper rail 41 becomes unsteady.
In a seat track mechanism for an automobile which supports a vehicle seat and allows the vehicle seat to be moved forward and rearward for adjustment, a pair of upper rails fixed to the bottom of the vehicle seat are supported by a pair of lower rails fixed to a vehicle floor to be slidable relative to the pair of lower rails, respectively. Although mainly a downward load due to the weight of the seated occupant and the weight of the vehicle seat itself acts on the pair of upper rails under normal usage conditions, an upward tensile load (pulling load) acts on a seatbelt anchor fixed to one of the pair of upper rails if the seatbelt is pulled strongly at a time of, e.g., collision of the vehicle. To prevent the upper rail, having the seatbelt anchor, from being detached from the associated lower rail to protect the seated occupant upon such an upward tensile load acting on the seatbelt anchor, the seat track mechanism is designed to have an anti-detaching structure preventing the upper rail from moving upward relative to the lower rail. A seat track mechanism with such an anti-detaching structure is known in the art in which a pair of anti-detaching flanges, such as the inward-extending flanges 40c of FIG. 16, are formed on the top side of a lower rail that has a box-shaped cross section and in which a corresponding pair of anti-detaching portions which come in contact with the pair of anti-detaching flanges of the lower rail from below are formed on the associated upper rail in a manner such as disclosed in Japanese unexamined patent publication NO. 2003-72432.
Due to the structure of a seatbelt, the pulling load that is imposed on the seatbelt anchor-bearing upper rail via the seatbelt and the seatbelt anchor acts on the seatbelt anchor-bearing upper rail in an upward direction slightly inclined to the vertical direction. In an upper rail provided in the widthwise center thereof with a portion having an inverted U-shaped cross section and further provided at the opposite ends of this central portion in the widthwise direction of the upper rail with two anti-detaching portions, respectively, i.e., in a type of upper rail having a hat-shaped (Ω-shaped) cross section such as disclosed in Japanese unexamined patent publication No. 2003-72432, there is a possibility of the upper rail being deformed in a manner to make a central portion of the upper rail become narrower (to narrow the width of the central portion of the upper rail) when the upper rail is acted upon by a pulling load in a direction obliquely upward via the anti-detaching portions. If this sort of deformation appears, the strength of the upper rail decreases or the upper rail becomes unsteady when the upper rail slides on the lower rail, and accordingly, it is necessary to take such measures as a measure to provide the aforementioned seatbelt anchor-bearing upper rail therein with a reinforcing member or the like which prevents this upper rail from being deformed.